Typically, most MLB teams carry two catchers during the bulk of the season: an everyday catcher and a back-up, or two catchers with playing time more or less evenly split depending on the opposing team. The Boston Red Sox carry three on their roster.
The catcher is usually the backbone of every team. He works alongside his coaches and pitchers to understand the psychology and behavioral tendencies of every opposing player. The role of the catcher is very mentally demanding. Throughout the entire 9-innings (assuming the game doesn’t go into extra innings), the catcher is responsible for playing a game of chess.